C H A P T E R O N E

INTRODUCTION

Microwave dielectric materials play a key role in global society with a wide range ofapplications from terrestrial and satellite communication including software radio, GPS,and DBS TV to environmental monitoring via satellites. In order to meet the specifica-tions of the current and future systems, improved or new microwave components basedon dedicated dielectric materials and new designs are required. The recent progress inmicrowave telecommunication, satellite broadcasting and intelligent transport systems(ITS) has resulted in an increasing demand for dielectric resonators (DRs), which are lowloss ceramic pucks used mainly in wireless communication devices. With the recentrevolution in mobile phone and satellite communication systems using microwaves asthe carrier, the research and development in the field of device miniaturization has beenone of the biggest challenges in contemporary Materials Science. This revolution isapparent on a daily basis in the ever increasing number of cell phone users. The recentadvances in materials development has led to these revolutionary changes in wirelesscommunication technology. Dielectric oxide ceramics have revolutionized the micro-wave wireless communication industry by reducing the size and cost of filter, oscillatorand antenna components in applications ranging from cellular phones to global position-ing systems. Wireless communication technology demands materials which have theirown specialized requirements and functions. The importance of miniaturization cannotbe overemphasized in any hand-held communication application and can be seen in thedramatic decrease in the size and weight of devices such as cell phones in recent years.This constant need for miniaturization provides a continuing driving force for thediscovery and development of increasingly sophisticated materials to perform the sameor improved function with decreased size and weight.

A DR is an electromagnetic component that exhibits resonance with useful propertiesfor a narrow range of frequencies. The resonance is similar to that of a circular hollowmetallic waveguide except for the boundary being defined by a large change in permit-tivity rather than by a conductor. Dielectric resonators generally consist of a puck ofceramic that has a high permittivity and a low dissipation factor. The resonant frequencyis determined by the overall physical dimensions of the puck and the permittivity of thematerial and its immediate surroundings. The key properties required for a DR are highquality factor (Q), high relative permittivity ("r) and near zero temperature coefficient ofresonant frequency (� f). An optimal DR that satisfies these three properties simulta-neously is difficult to achieve in a particular material.

In the early microwave systems, bulk metallic cavities were used as resonators, but werehuge and not integrated with microwave integrated circuits (MICs). On the other hand,stripline resonators have a poor quality factor and poor temperature stability resulting inthe instability of the circuit. Hence the importance of DRs, which are easily integratedwith MICs with low loss and with thermally stable frequency, especially at mm wave-lengths. Most of the microwave-based device systems are located in the frequency range

Dielectric Materials for Wireless Communications � 2008 Elsevier Ltd.

ISBN-13: 978-0-08-045330-9 All rights reserved.

1

300 MHz–300 GHz as shown in Figure 1.1. Technological improvements in DRs havecontributed to considerable advancements in modern wireless communications. CeramicDRs have the advantage of being more miniaturized as compared to traditional microwavecavities, and have a significantly higher quality factor. DRs have replaced cavity resonatorsin most microwave and millimeter-wave applications for reasons of cost, dimension, mass,stability, efficiency, tenability, ruggedness and ease of use. In addition, the temperaturevariation of the resonant frequency of DRs can be engineered to a desired value to meetcircuit designer’s requirements. Functioning as important components in communicationcircuits, DRs can create and filter frequencies in oscillators, amplifiers and tuners. In orderto respond to the requirement for increased channel capacity in ground-based cellular andsatellite communications, new devices with superior performance must be developed. Thesystem performance is closely related to material properties. In microwave communica-tions, DR filters are used to discriminate between wanted and unwanted signal frequenciesfrom the transmitted and received signals. The desired frequency is extracted and detectedto maintain a strong signal-to-noise ratio. For clarity, it is also critical that the wanted signalfrequencies are not affected by seasonal temperature changes.

The low permittivity ceramics are used for millimeter-wave communication and also assubstrates for microwave integrated circuits. The medium "r ceramics with permittivity inthe range 25–50 are used for satellite communications and in cell phone base stations. Thehigh "r materials are used in mobile phones, where miniaturization of the device is veryimportant. For millimeter-wave and substrate application, a temperature-stable lowpermittivity and high Q (low loss) materials are required for high speed signal transmissionwith minimum attenuation.

The term ‘‘dielectric resonator’’ first appeared in 1939, when Richtmeyer of StanfordUniversity showed that a suitably shaped dielectric piece can function as a microwave

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Figure 1.1 Microwave spectrum and applications.

2 Chapter 1 Introduction

resonator [1]. However, it took more than 20 years to generate further interest on DRsand to test Richtmeyer’s prediction experimentally. In 1953, Schlicke [2] reported onsuper high permittivity materials (�1000 or more) and their applications as capacitors atrelatively low RF frequencies. In the early 1960s, Okaya and Barash from ColumbiaUniversity rediscovered DRs while working on rutile single crystals [3, 4]. Okaya andBarash [3, 4] measured the permittivity and Q of TiO2 single crystals at room tempera-ture down to 50 K in the microwave frequency range, using the commensurate trans-mission line technique [4]. Later several authors developed methods for measuring the "r,quality factor (Q) and � f of DRs. These methods are discussed in Chapter 2. In the early1960s, Cohen and his co-workers [5] from Rantec Corporation performed the firstextensive theoretical and experimental evolution of DR. Rutile ceramics were used forthe experiments that had an isotropic permittivity of about 100. The TiO2 has a poor(þ450 ppm/�C) stability of resonant frequency that prevented its commercial exploita-tion. The first microwave filter using TiO2 ceramics was proposed by Cohen in 1968[6, 7]. But this filter was not useful for practical applications because of its high � f. A realbreakthrough in DR ceramic technology occurred in the early 1970s, when the firsttemperature-stable, low loss barium tetratitanate (BaTi4O9) ceramics were developed byMasse et al. [8]. Later, barium nanotitanate (Ba2Ti9O20) with improved performance wasreported by Bell Laboratories [9]. The next breakthrough came from Japan when MurataManufacturing Company produced (Zr,Sn)TiO4 ceramics [10, 11]. They offered adjus-table compositions so that temperature coefficients could be varied between þ10 and�10 ppm/�C. Later, in 1975, Wakino et al. realized the miniaturization of the DR-basedfilters and oscillators [12]. Since then extensive theoretical and experimental work anddevelopment of several DR materials has occurred. This early work resulted in the actualuse of DRs as microwave components. Commercial production of DRs started in theearly 1980s. The number of papers published and patents filed on the science andtechnology of DRs increased considerably over the years as shown in Figure 1.2.There are about 2300 low loss dielectric materials reported in the literature (see Appendix 2).More than 5000 papers have been published and over 1000 patents were filed on DRmaterials and devices. However, with only a limited number of useful dielectric ceramicmaterials to choose from, the electronic industry is constantly searching for new materialsthat are easily affordable for manufacture.

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Figure 1.2 (a) The number of papers published on dielectric resonator materials andtechnology versus year of publishing (b) Number of patents filed versus year.

Introduction 3

Richtmeyer [1] in 1939 theoretically predicted that a piece of dielectric with regulargeometry and high "r can confine electromagnetic energy within itself, but still be proneto energy loss due to radiation. It was found that through total multiple internalreflections, a piece of high "r dielectric can confine microwave energy at a few discretefrequencies, provided the energy is fed in the appropriate direction (see Figure 1.3). Ifthe transverse dimensions of the sample are comparable to the wavelength of themicrowave, then certain field distributions or modes will satisfy Maxwell’s equationsand boundary conditions. The reflection coefficient approaches unity as "r approachesinfinity. In the microwave frequency range, free space wavelength (�c) is in centimetersand hence the wavelength (�g) inside the dielectric will be in millimeters only when thevalue of "r is in the range 20–100. To get resonance, dimensions of the dielectric must beof the same order (in millimeters). Still larger "r gives higher confinement of energy,reduced radiation loss and better miniaturization. However, high "r will result in higherdielectric losses because of inherent material properties. When exposed to free space, aDR can also radiate microwave energy when it is fed suitably and can be used as efficientradiators, called Dielectric Resonator Antennas (DRA). A DR with finite values of "r

prevents 100% reflection from the air/dielectric boundary and hence some field willalways exist in the vicinity of the dielectric. This is of great advantage since it enables oneto couple microwave power easily to the DR by matching the field pattern of thecoupling elements to that of the DR. Figure 1.4 (a) and (b) illustrates the variation ofelectric and magnetic fields inside a dielectric (Ca5Nb2TiO12 ceramic puck with "r = 48)kept inside a copper cavity and simulated using a three-dimensional transmission linematrix modeling method [13].

The size of a DR is considerably smaller than the size of an empty resonant cavityoperating at the same frequency, provided the relative permittivity ("r) of the material is

Figure 1.3 Schematic sketch of total multiple internal reflections in a high "r dielectric piece.

4 Chapter 1 Introduction

substantially higher than unity. Higher "r shrinks overall circuit/device size proportionalto (1/"r)

1/2. For example, a circuit is compressed by a factor of six when a high Q

ceramic with "r = 36 is substituted for a high Q air cavity "r = 1. The shape of a DR isusually a solid cylinder but can also be tubular, spherical and parallelepiped. Figure 1.5shows some of the low loss dielectric pucks made at the author’s laboratory. A commonlyused resonant mode of a cylindrical DR is TE01�. At resonant frequency, electromagneticfields inside a resonator store energy equally in electric and magnetic fields. When "r isabout 40, more than 95% of the stored electric energy and over 60% stored magneticenergy are located within the dielectric cylinder. The remaining energy is distributed inthe air around the resonator, decaying rapidly with distance away from the resonatorboundary. The DR can be incorporated into a microwave network by exciting it withmicrostrip transmission lines, as shown in Figure 1.6. The distance between the resonatorand the microstrip conductor determines the amount of coupling. In order to preventlosses due to radiation, the entire device is usually enclosed in a metallic shielding box.

High Q minimizes circuit insertion losses and can be used as a highly selective circuit.In addition, high Q suppresses the electrical noise in oscillator devices. Although severalmanufacturers may produce similar components for the same application, there are subtledifferences in circuit design, construction and packaging. Since frequency drift of adevice is a consequence of the overall thermal expansion of its unique combination of

Figure 1.5 Picture of dielectric ceramic packs developed at the author’s laboratory (seeColor Plate section).

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Figure 1.4 Variation of (a) electric and (b) magnetic fields of TE01d resonance mode of aCa5Nb2TiO12 ceramic resonator with "r = 48 (after Ref. [13]) (see Color Plate section).

Introduction 5

Figure 1.6 Dielectric resonator mounted on amicrostrip.

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Figure 1.7 The variation of the relative density, "r, Qf and � f of Ba(Sm1/2Nb1/2)O3 ceramicversus calcination temperature. Sintering temperature1550�C for 2 hours (after Ref. [16]).

6 Chapter 1 Introduction

construction materials, each design requires a slightly different � f for temperature com-pensation. Typically, ceramics with a specific � f in the range 15 to �15 ppm/�C areselected. In ceramic production, � f and "r specifications must be held in demandingtolerances �1%.

Oxide ceramics are critical elements in these microwave circuits, and a full under-standing of their crystal chemistry is fundamental to future development. Properties ofmicrowave ceramics critically depend on several parameters, such as purity of startingmaterials, calcination temperature and duration, shaping method and sintering tempera-ture and durations. Design of the heating/cooling schedule requires knowledge offormation mechanism of various phases in multicomponent systems. The starting pow-ders must sinter to high density to get optimum electrical properties. Figures 1.7–1.10show typical examples of the effect of calcination and sintering temperatures and theirdurations on the density and microwave dielectric properties of ceramic DRs. Themicrowave ceramics are usually optimized for the best density for which the "r and Qfare normally the best. Powders prepared by solid state methods require a higher sinteringtemperature as compared to those prepared by chemical methods. However, the sinter-ing temperature can be lowered by using sintering aids. High Qf can be achieved only inceramics with fine grains and/or homogeneous microstructures. The ceramics withdiscontinuous and excess grain growth exhibit poor performance. Incomplete densifica-tion is mainly due to discontinuous grain growth. Due to rapid and discontinuous graingrowth, porosity is trapped in. If the distance from these trapped pores to the path of thefast material transport or grain boundaries is large, the rate of material transport to close

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Introduction 7

the pores will be limited. The materials will be porous and the dielectric and mechanicalproperties will be poor. Densification can be promoted by the use of additives. Additivesaid fabrication by allowing densification to occur at lower temperatures in shorter timesor by inhibiting discontinuous grain growth and allowing pore elimination to proceed tocompletion. Use of these additives is for most part empirical, and experimental verifica-tion of their role is impossible usually limited to the grain boundaries. At least twotheories were advanced to explain the effect of these additives. (a) Liquid phase assistance.Additive with a lower MPt is used for densification by melting and coating the ceramicparticles, rapid dissolution and transfer of the base material to fill the interparticle spaces,which leads to enhanced densification. The additive has to be fairly soluble in the baseceramic and several wt% of the additive is necessary to provide sufficient liquid phase tocoat all the powder particles. (b) Solid solution effect. Addition of a solute with adifferent valency enhances bulk material transport due to the introduction of vacanciesin the ceramic. These vacancies render the diffusion coefficient extrinsic by makingthermal vacancy concentration insignificant up to a certain temperature. If added insufficient quantity, it can significantly alter the rate of material transport through the solidphase. Sintering aids affect ceramics in many ways by changing the density, microstruc-ture, defect structure and possibly crystal structure. These changes brought about by thesintering aids affect the resulting dielectric properties. The density "r, Q and � f are allaffected by the additives. A higher relative density results in a higher "r. The selection ofproper additives, their optimum quantity and optimum processing conditions are effec-tive in enhancing the quality factor.

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Figure 1.9 The variation of the relative density, "r, Qf and � f of Ba(Sm1/2Nb1/2)O3 ceramicversus sintering temperature (after Ref. [16]).

8 Chapter 1 Introduction

Although the title of the book is Dielectric Materials for Wireless Communication, it maybe noted that the materials discussed in this book are useful for several other applicationssuch as capacitors and gate dielectrics. Although DRs are so promising in practicalapplications, surprisingly no reference or edited books summarizing the research resultson low loss dielectric materials are available. The first book on DRs by Kajfezz andGuillon [14] and the book on DRA by Luk and Leung [15] essentially deal withelectromagnetic theory and applications and only discuss DR materials in passing.During the last 25 years, scientists the world over have developed a large number ofnew low loss dielectric materials and improved the properties of existing materials.Actually a few thousand research papers have been published on the preparation,characterization and properties of DRs. However, the data of this multitude of veryuseful materials are scattered. Hence it is the purpose of this book to bring the data andscience of these several useful materials together which will be of immense help toresearchers and technologists the world over. The book describes the preparation,characterization and properties of important DR materials and how one can tailor theproperties to meet the requirements of the design engineer.

The topics covered in the book includes factors affecting the dielectric properties,measurement of dielectric properties, important low loss dielectric material systems suchas perovskites, tungsten bronze type materials, materials in BaO–TiO2 system,(Zr,Sn)TiO4, alumina, rutile, AnBn–1O3n type materials, LTCC, etc. The book also hasa data table listing the reported low loss dielectric materials with properties and referencesarranged in the order of increasing permittivity.

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Figure 1.10 The variation of the relative density, "r, Qf and � f of Ba(Sm1/2Nb1/2)O3ceramic versus sintering duration on sintering at1575�C (after Ref. [16]).

Introduction 9

Filling an existing need, Dielectric Materials for Wireless Communication is the firstcomprehensive book in this rapidly growing field.

REFERENCES

[1] R. D. Richtmeyer. Dielectric resonators. J. Appl. Phys. 15 (1939)391–398.[2] H. M. Schlicke. Quasidegenerate modes in high "r dielectric cavities. J. Appl. Phys. 24

(1953)187–191.[3] A. Okaya. The rutile microwave resonator. Proc. IRE 48 (1960) 1921.[4] A. Okaya and L. F. Barash.The dielectric microwave resonator. Proc. IRE 50 (1962) 2081–2092.[5] S. B. Cohen. Microwave band pass filters containing high Q dielectric resonators. IEEE

Trans. Microw. Theory and Techniques MTT-16 (1968)1628–1629.[6] S. B. Cohen. Microwave filters containing high Q dielectric resonators. IEEE MTT-Symp.

Dig. (1965)49–53.[7] S. B. Cohen. Microwave band pass filters containing high Q dielectric resonators. IEEE

Trans. Microw. Theory and Techniques. MTT-33 (1985)586–592.[8] D. J. Masse, R. A. Purcel, D. W. Ready, E. A. Maguire and C. P. Hartwig. A new low loss

high K temperature compensated dielectric for microwave applications. Proc. IEEE 59(1971)1628–1629.

[9] J. K. Plourde, D. F. Limn, H. M. O’Bryan, and J. Thomson Jr. Ba2Ti9O20 as a microwaveresonator. J. Am. Ceram. Soc. 58 (1975)418–420.

[10] K. Wakino, M. Katsube, H. Tamura, T. Nishikawa, and Y. Ishikawa. Microwave dielectricmaterials (Japanese). In IEEE Four Joint Cov. Rec. (1977) paper No. 235.

[11] K. Wakino, T. Nishikawa, H. Tamura, and Y. Ishikawa. Microwave band pass filterscontaining dielectric resonator with improved temperature stability and spurious response.IEEE MTT-S Int. Microw. Symp. Dig. (1975)63–65.

[12] K. Wakino, T. Nishikawa, S. Tamura, and Y. Ishikawa. Microwave band pass filters contain-ing dielectric resonators with improved temperature stability and spurious response. IEEEMTT-S. Int. Symp. Dig. (1975)63–65.

[13] P. V. Bijumon, M. T. Sebastian, and P. Mohanan. Experimental investigations and threedimensional transmission line matrix simulation of Ca5–xAxB2TiO12 (A=Mg, Zn, Ni and Co;B=Nb and Ta) ceramic resonators. J. Appl. Phys. 98 (2005)124105.

[14] D. Kajfezz and P. Guillon (Eds). Dielectric Resonators, 2nd Edition. Noble PublishingCorporation, Atlanta, US (1998).

[15] K. M. Luk and K. W. Leung (Eds). Dielectric Resonator Antennas. Research Studies PressLtd, Baldock, Hertfordshire, UK (2002).

[16] L. A. Khalam. The A(B01/2B00

1/2)O3 {A=Ba, Sr, Ca, Mg; B=Re, and B=Nb,Ta} microwaveceramics for wireless communications. Ph.D. Thesis, Kerala University, India (2007).

10 Chapter 1 Introduction